An Internet Protocol Multimedia Subsystem (IMS) subscriber can be registered, as user of the IMS network, on multiple user equipment. Thereto, a User Agent associated with the subscriber has registered itself with contact addresses for the respective user equipment. The subscriber will be registered in a Serving Call Session Control Function (S-CSCF) entity. In the S-CSCF entity, the contact addresses for these user equipment are stored, for as long as the registration of each user equipment is valid. The contact addresses constitute addresses identifying or related to these user equipment, facilitating the S-CSCF entity to forward a message to these user equipment. An IMS-subscriber can subscribe to multiple services. These services are executed by service logic located in one or multiple Session Initiation Protocol Application Servers (SIP-AS's). In the S-CSCF entity, Initial Filter Criteria (IFC) for an IMS subscriber, executed by the S-CSCF entity, point to these SIP-AS's. The IFC consists of a set of regular expressions, analysed and acted upon at call set-up or at registration. The IFCs are stored in the S-CSCF entity during registration. For each subscriber there may be a specific set of IFC.
After execution of the services in the SIP-AS's, the S-CSCF entity distributes the SIP Invite to the registered devices, e.g. by means of forking.